Oxides of nitrogen (NOx) are undesirable byproducts of combustion of a fuel in air. Some fuels, such as coal and biomass provide additional nitrogen and can be more problematic. Unfortunately, combustion of inexpensive fuels such as coal, biomass, and waste may tend to produce the most NOx. Regulations and general concerns for air quality have caused manufacturers and operators of combustion systems to seek ways to decrease emissions of NOx from combustion processes.
One approach to decrease the output of thermal NOx is to decrease peak-combustion reaction temperature. Another approach to decrease the output of NOx is to convert NOx present in post-combustion gases into molecular nitrogen, N2. Since NOx is an oxidized form of nitrogen, conversion of NOx to N2 is referred to as nitrogen reduction. Selective nitrogen reduction processes including selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are used to chemically reduce oxides of nitrogen (NOx) to molecular nitrogen, N2.
NOx typically includes NO and NO2, but at high temperatures is usually dominated by NO. In SNCR, a nitrogen compound such as ammonia (NH3), urea (NH2CONH2), or another reagent is injected into hot (but not too hot) combustion fluids, such as in a firebox or boiler. If urea is injected, it reacts to form ammonia according to reaction (1):NH2CONH2+½ O2→2 NH3+CO2  (1)
The nitrogen reduction reaction may be expressed as:4 NO +4 NH3 + O2 →4 N2 +6 H2O  (2)
The mechanism for reaction (2) involves the formation of intermediate .NH2 radicals that react with NO to form the reaction products N2 and H2O.
One complication with the chemistries described above relates to temperature. At temperatures above 1093° C., ammonia decomposes to form NO according to reaction (3):4 NH3 +5 O2 →4 NO +6 H2O  (3)
Other complications to operation of SCR/SNCR systems relate to non-uniform NOx distribution in a combustion volume or flue gas and delivery of an appropriate amount of reducing agent to the NOx distribution. Since central regions of fireboxes and furnaces tend to be hotter than regions near firebox and furnace walls, more NO tends to be formed near the center. Thus, uniform distribution of NH3 across a combustion volume will not result in uniform reduction in NOx. Moreover, it can be difficult to distribute NH3 to areas where it is needed.
Generally, existing SCR/SNCR systems suffer from ammonia slip (passage of unreacted ammonia out a flue) and lower than theoretical efficiency (equilibrium) with respect to removal of NOx.
What is needed is a technology that can improve performance and/or reduce costs of SCR and SNCR systems.